Download The PV Combiner Multiple Strings May Need Combiner

still on the roof
STILL
ON THE
ROOF
Photo 1. PV source-circuit combiner with fuses
by John Wiles
I
n our top-to-bottom perspective of a PV system, we
need to look at one more component usually located
on the roof. This is the PV source-circuit combiner
and it will be followed in the dc circuit by the PV dc
disconnecting means.
are no hard and fast rules relating the need for a dc combiner to a specific number of modules in an array.
Multiple Strings May Need Combiner
The rated output voltage of the PV modules and the
inverter dc input characteristics determine how many
The PV Combiner
modules may be connected in a series string. See “PV
The PV source-circuit combiner is found on larger residen- Math” in the January-February 2009 IAEI News. The
tial systems and on most large commercial systems. PV power rating of each module and the number of modsystems that have a dc rating above about 6 kW may have ules in a string determine the power rating of a string.
sufficiently large numbers of modules that more than two The desired array power rating and the power rating of
strings of modules are required to get the desired array the inverter determine how many strings can be conpower. Since module voltages range widely and module nected in parallel. Normally two strings can be connectpower ratings can vary from 40 watts to 300 watts, there ed in parallel without requiring a combiner containing
32 IAEI NEWS March.April 2009
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still on the roof
Photo 2. PV source-circuit combiner with circuit breakers—poor workmanship
overcurrent devices [See 690.9(A) Ex]. If more than two
strings are needed, then overcurrent protection on each
string may be required and these overcurrent devices are
placed in a PV source-circuit combiner. See “Questions
from the AHJ—To Fuse or Not to Fuse” in the MayJune 2008 IAEI News for the details.
A combiner may use either fuses (typically on highvoltage, utility-interactive systems) or circuit breakers
(used on systems operating at a nominal 48 volts or below). [See photos 1, 2, and 3].
It should be noted that the combiners shown in photos 1 and 3 have exposed circuit terminals and busbars
near the overcurrent devices. They are not dead front
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when opened, and voltages on the exposed terminals and
busbars may approach 600 volts on many systems in cold
weather. Although not dead front, these combiners meet
the “intent” of the NEC where a tool is required to get
access to energized surfaces (terminals and busbars). In
these cases, the combiners have screw-on covers and the
tool required to open the combiner is a screwdriver. NEC2008 requires that combiners be listed and UL has determined that the listing must be to UL Standard 1741 (the
PV inverter standard) [690.4(D)]. Although listing is required, UL 1741 has not yet been modified to specifically
require that combiners be dead front. Some of the newer
units are dead front, and eventually that requirement will
March.April 2009 IAEI NEWS
33
still on the roof
be in the standard. See photo 4 of a dead front unit that
has terminal covers made of clear plastic.
Wiring from the PV Array to the
PV Disconnect
Photo 3. PV source-circuit combiner on large system
Although the conductors between the modules and the
return circuit from one end of a string to the other are
permitted to be single conductor cables in free air, as
soon as these circuits leave the array location they must
transition to a standard chapter 3 wiring method. That
wiring method must be suitable for the hot, wet environment found on roofs, and sunlight resistance is also
a must. Electrical metallic tubing (EMT) is frequently
used. The ampacity of the conductors is based on the
short-circuit current being carried in that circuit and
must be corrected for the conditions of use. In many
cases, terminal temperature limitations on combiners or
fused disconnects may dictate further ampacity corrections. See the “The Nature of the PV Module: Limited
Currents Have Benefits and Drawbacks,” SeptemberOctober 2007 IAEI News for more details.
An equipment grounding conductor should be run
with the circuit conductors in the conduit. Where the
PV source circuits are unfused, the size under the 2005
NEC was based on 125 percent of the rated short-circuit current (Isc) for the circuit. In the 2008 NEC, Isc is
used directly in Table 250.122 to select an equipmentgrounding conductor. The reduction in size of the dc
equipment-grounding conductor is due to the 2008
NEC requirement that nearly all
PV systems have ground-fault
detectors that will limit groundfault currents in the equipment
grounding conductors. On systems with PV source or output
circuit fuses, the normal procedure of using the fuse value in
Table 250.122 is used (690.45).
In many systems, the equipment grounding conductors may
be as small as 14 AWG between
the PV modules. However, in areas where winds, snow, ice and
other environmental factors are
significant, a larger equipment
grounding conductor should be
considered to provide additional
mechanical protection (690.46).
The dc PV Disconnect
Photo 4. Dead front PV source-circuit combiner (photo credit AMTec Solar)
The dc PV disconnecting means (PV disconnect) should
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still on the roof
Photo 5. PV dc disconnecting means
be installed in a readily accessible location, either inside
or outside the building at the point of first penetration
of the conductors (690.14) (photo 5). Since Section
690.31(E) allows the PV source or output conductors
to penetrate the building surface on the roof (if they
are routed in a metal raceway inside the building), it
appears that the PV disconnect can be mounted inside
the building in any readily accessible location. However,
this NEC allowance may not be the safest option or even
very clearly defined in the national Code.
This parallel wording of 690.14(C)(1) to the requirements for the ac service disconnecting means 230.70(A)
(1) may need further examination since in the world of ac
utility-power, removal of the ac revenue meter can effectively disable the ac power in a structure where the ac service disconnect is inside a locked structure. With a dc PV
disconnect inside a locked structure, the readily accessible
definition may not be appropriate. Many jurisdictions require that the PV disconnect be located within sight of the
ac service disconnect or meter on residences. This is usually
on the outside of the building. On commercial buildings,
the PV system may be some distance from the ac service
disconnect, and a directory may be used to show the location of all disconnects, both ac and dc (705.10).
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The disconnect should break all ungrounded conductors, but should not
open a grounded conductor. Grounded
conductors in PV systems may be either
the negative or positive source-circuit
conductors and should have white insulation, or where larger than 6 AWG, be
marked with a white marking. The type
of module used determines which circuit
conductor should be grounded and the
inverter must be compatible with that
polarity of grounded conductor. The dc
bonding jumper in a utility-interactive
PV system is commonly inside the inverter and is a part of the ground-fault
detection/interruption systems required
by 690.5.
If the grounded source-circuit conductor is opened by the switch in the
disconnect, the marked grounded conductor becomes ungrounded and may
be energized with respect to ground up
to the open-circuit voltage of the system. This represents an unsafe condition for people servicing the PV array
and for that reason the Code prohibits
the use of disconnects, breakers or fuses in grounded
PV dc conductors unless they are part of an automatic
ground-fault detection/interruption system (690.13).
Photo 6 shows the front of a PV dc disconnect with
the labels required by 690.17 and 690. 53. The 690.17
warning is required because the load terminals of this
disconnect are connected to the inverter dc input
which may be energized for up to five minutes after
Photo 6. Disconnect Labels
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still on the roof
with the inverter ac output connected to the load side.
No warning label is required because, when the disconnect is opened, the inverter ceases to produce power
within a fraction of a second and the exposed load side
terminals pose no shock hazard.
Summary
Attention to the Code requirements in 690 and other articles plus an understanding of PV equipment and how
power flows in a PV system should enable these systems
to be installed and operated in a safe manner. The utility-interactive inverter is next on our top-to-bottom tour
of the PV system.
For Additional Information
Photo 7. Line and load connections are important
the disconnect has been opened. The filter and energy
storage capacitors in the inverter will be discharged after this time. The 690.53 label with the dc voltage and
current ratings will allow the AHJ to determine if the
correct cables have been installed.
Power flow in a PV system is from the PV array
through the dc PV disconnect, the inverter, the ac disconnect and finally to the grid. This power flow sometimes confuses installers on how to properly connect the
dc and ac disconnects. Note the upper line-side terminals on the disconnect shown in photo 7 are covered
by an insulated cover. Also note the switchblades, the
fuse holder terminals (if any), and the load-side lower
terminals are exposed and easily touched. A general
safety rule is that the most dangerous circuits should be
connected to the protected line-side terminals. If this
is done, it is less likely that energized terminal connections will be accidentally touched when the door of the
disconnect is open. In the PV dc disconnect, the PV
source or output circuits should always be connected
to the line-side terminals. The dc input to the inverter
is connected to the load-side terminals and the 690.17
warning label is required as shown in photo 6.
For the ac PV disconnect the circuit connected to the
utility source should be the line side of the disconnect
If this article has raised questions, do not hesitate to
contact the author by phone or e-mail. E-mail: jwiles@
nmsu.edu Phone: 575-646-6105
A color copy of the latest version (1.9) of the 150-page,
Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices, written by the author, may
be downloaded from this web site: http://www.nmsu.
edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html
The Southwest Technology Development Institute web
site maintains a PV Systems Inspector/Installer Checklist
and all copies of the previous “Perspectives on PV” articles for easy downloading. Copies of “Code Corner” written by the author and published in Home Power Magazine
over the last 10 years are also available on this web site:
http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/
Codes-Stds.html
The author makes 6–8 hour presentations on “PV
Systems and the NEC” to groups of 60 or more inspectors, electricians, electrical contractors, and PV professionals for a very nominal cost on an as-requested basis.
A schedule of future presentations can be found on the
IEE/SWTDI web site.
John Wiles works at the Institute for Energy and
the Environment (IEE) (formerly the Southwest Technology Development Institute) at New Mexico State
University. IEE has a contract with the US Department of Energy to provide engineering support to the
PV industry and to provide that industry, electrical
contractors, electricians, and electrical inspectors with
a focal point for Code issues related to PV systems. He serves as the secretary of the PV Industry Forum that submitted 54 proposals for the
2011 NEC. He provides draft comments to NFPA for Article 690 in
the NEC Handbook. As an old solar pioneer, he lived for 16 years in a
stand-alone PV-power home in suburbia with his wife, two dogs, and a
cat—permitted and inspected, of course. The PV system on his home is a
5 kW (dc) utility-interactive with a full-house battery back up.
This work was supported by the United States Department of Energy
under Contract DE-FC 36-05-G015149
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